1. Field
Apparatus and methods described in this document relate to switching circuits and, more specifically, to capacitor switching circuits for tuning oscillator frequency in communication equipment.
2. Background
Tunable frequency generators are used in many different electronic devices. Wireless communication devices, for example, use frequency generators for upconversion of transmitted signals to intermediate and RF frequencies, and for downconversion of received signals to intermediate and baseband frequencies. Because operating frequencies vary, the generators' frequencies need to be tunable.
Frequency coverage required for multiple communication standards and multiple bands typically necessitates wide tuning range oscillators, such as voltage controlled oscillators (VCOs) and digitally controlled oscillators (DCOs). The extent of an oscillator's tuning range is one important performance parameter. It is often desirable to increase the tuning range, for example, in order to cover multiple bands.
Other performance criteria of tunable oscillators include phase noise performance, power consumption, and size. The different performance criteria are sometimes competing.
Conventional tunable oscillators may be tuned, for example, by switching capacitors into and out of the oscillator inductance-capacitance (LC) tank. The highest frequency and the accuracy of tuning of such a tunable oscillator may both be limited by the parasitic capacitances in the capacitance switching circuits. Therefore, it is desirable to reduce the parasitic capacitances associated with the tuning circuits.
Advancements in integrated circuit technology have enabled the shrinking of device sizes or scales, and thereby the reduction in supply voltage levels. Voltage swing requirements across different terminals of these devices, however, may not scale proportionally with device sizes. Stringent phase noise requirements in high performance oscillators for wireless applications may require relatively large voltage swings across the LC tank. For example, oscillator tank voltage swing may need to be as large as three volts peak-to-peak, to meet the phase noise specification requirements applicable to the Personal Communication Service (PCS) frequency band in CDMA 1× mode.
Large voltage swings may excessively stress transistor devices and reduce the devices' lifetime expectations, or cause outright device failures. Hence, reliability and durability concerns tend to play an important part in driving design decisions in nanometer complimentary metal oxide semiconductor (CMOS) processes.
One of the important blocks of a tunable oscillator may be a coarse frequency tuning block, with an array or bank of binary-weighted capacitors with switches. FIG. 1 illustrates selected parts of one exemplary element 100 of a capacitor bank. The parts include a transistor switch 105, capacitors 130 and 135, and resistors 110, 115, and 120.
The element is configured so that the state of the transistor 105 is controlled by the complimentary control signal voltages b0 and b0. When b0 is low and b0 is high, the transistor 105 is in the conducting (low impedance) state, placing the series combination of the capacitors 130/135 effectively in the tank circuit of the tunable oscillator. When b0 is high and b0 is low, the transistor 105 is in the non-conducting (high impedance) state, effectively removing the series combination of the capacitors 130/135 from the tank circuit of the tunable oscillator. Assuming that the parasitic capacitance between the drain and source of the transistor 105 is small compared to the capacitances of the capacitors 130/135, it is the parasitic capacitance that dominates the total capacitance contributed to the tank circuit by the element 100. Therefore, it is desirable to keep the parasitic capacitance low, so that the effect of the element 100 on the tank circuit is reduced in the non-conducting state.
In the off state of the transistor 105, the maximum potential differences between source and drain (VSD), source and gate (VSG), and drain and gate (VDG) depend on the control voltages b0 and b0 and the voltages across the tank circuit of the tunable oscillator. Assuming that (1) the voltage across the tank of the oscillator varies between ground and 2×VDD potentials, and (2) the control signal voltages b0 and b0 also vary between ground and VDD potentials, the magnitude of the potential differences VSD, VSG, and VDG reaches (2×VDD). As noted above, this may unduly stress the transistor 105, particularly in small scale nanometer designs, and thus cause reductions in the reliability and durability performance metrics of the transistor 105. The breakdown problem may be present in 65 nanometer scale designs, and is likely to aggravate as technology progresses to 45 nanometer scale, 32 nanometer scale, and deeper into the sub-micron region.
One way to improve the reliability and durability of the transistor switches is to use thick oxide devices in the capacitor bank transistor switches, such as the transistor 105. Thick oxide transistors, as their name implies, have thicker oxide in their gate (as compared to thin oxide transistors), and hence can sustain higher gate-to-source and gate-to-drain voltages before breaking down. Although the potential differences across the transistor nodes remain the same with the thick oxide approach as with the thin oxide approach, the reliability and durability are improved because the thicker oxide breaks down at higher voltages. This advantage, however, comes at a cost: the thick oxide transistors also have large parasitic capacitances in the off (high impedance) state, which directly affects the tuning range. As discussed above, this is undesirable, especially in wide tuning range oscillators.
Therefore, there is a need in the art to improve reliability and durability of tunable capacitor banks, and particularly to improve reliability and durability of the transistor switches of such banks. There is a further need in the art to reduce the voltage stress on the individual transistor switches in capacitor banks of tunable oscillators. There is also a need in the art to reduce the voltage stress on the individual transistor switches in capacitor banks of tunable oscillators without unduly increasing parasitic capacitances of the transistors.